Oriales García Rubio knows how it feels to want more. When she was a girl in central Cuba in the 1930s, her family of nine lived in a one-room house with a dirt floor. Her dolls were Coke bottles dressed in rags. She dreamed of becoming an actress. Instead she married a security guard, moved with him to the U.S. and found work as a hotel maid. Her husband got a job as a bartender while starting a series of failed businesses—a vegetable stand, a dry cleaner, a grocery. They never had much. But their house had a real floor. Their daughters had real dolls. They sent all four of their children to college to chase their own dreams.

That’s why on the morning of Dec. 21, she called her youngest son, Marco Antonio Rubio, the 41-year-old Senator from Florida and great Hispanic hope of the Republican Party—or, as she calls him, Tony. She got his voice mail. “Tony, some loving advice from the person who cares for you most in the world,” she said in Spanish. “Don’t mess with the immigrants, my son. Please, don’t mess with them.” She reminded him that undocumented Americans—los pobrecitos, she called them, the poor things—work hard and get treated horribly. “They’re human beings just like us, and they came for the same reasons we came. To work. To improve their lives. So please, don’t mess with them.”

Rubio comes from a family of immigrants and married into another family of immigrants and lives in a neighborhood of immigrants, West Miami, the bilingual bedroom community where he came of age and began his dazzling ascent from city commissioner to state house speaker to U.S. Senator. Now, just two years after he arrived in Washington, the charismatic conservative often hailed as the Tea Party’s answer to Barack Obama has emerged as the most influential voice in the national debate over immigration reform. He’s also the key player in his party’s efforts to make up to Hispanic voters after a disastrous 2012 campaign featuring Republican candidates who proposed electric fences and alligators along the southern border, as well as Mitt Romney’s suggestion of “self-deportation” for the nation’s 11 million undocumented immigrants. GOP leaders know they have a demographic problem. They hope Rubio can help provide the solution, which is why they’ve chosen him to deliver the response to Obama’s State of the Union address on Feb. 12—in English and Spanish.

But while Rubio is a child of immigrants, he’s also a child of the conservative movement, an ambitious ideologue and former political operative who speaks partisan Republican with the fluency of a native. (Romney, by contrast, spoke it as a second language.) Like Paul Ryan, a potential 2016 rival, he’s part of a new generation of lean and hungry conservatives who grew up in the antigovernment Reagan era and entered politics after the scorched-earth Gingrich revolution. Bipartisan compromise is not usually his thing.

So he’s navigating a borderland of his own. He has endorsed a path to citizenship that he once derided as “code for amnesty,” risking a backlash from many loyal supporters who see los pobrecitos as freeloaders. But he has also pushed to make that path more arduous, demanding much tougher enforcement first, insisting he won’t get into a who-can-be-nicest bidding war with Obama and pledging to walk away from reform if the final legislation doesn’t reflect conservative principles. In an hour-long Feb. 1 interview with TIME, he emphasized that the undocumented have no right to stay in the U.S., vowed to oppose any bill that rewards them for breaking the law and defended the motives of hard-line “shamnesty” critics who say illegal immigrants are taking taxpayers for a ride. “Someone’s violated the law, and they’re receiving taxpayer benefits? That’s a legitimate reason to be upset,” Rubio says.

It’s a thin, hard line to walk: between the Republican establishment and the base, between compassion and the rule of law, between family and politics. And Rubio is walking it on an issue no politician has cracked in nearly two decades while testing the support of the grassroots Tea Party conservatives he will need if he seeks the White House in 2016. So far, though, he seems to be succeeding. After helping to craft bipartisan reforms in the Senate, he has served as their chief spokesman on right-wing radio and Fox News, getting remarkably sympathetic hearings from Rush Limbaugh, Sean Hannity and other talkers whose antiamnesty crusades helped kill similar efforts in the George W. Bush era. Almost all of them have praised his courage—Limbaugh called his work “admirable,” like a Pope granting absolution—and the backlash has yet to materialize. “I don’t know anyone else who could have broken through the conservative sound barrier on immigration,” says American Conservative Union chairman Al Cardenas, a Miami lawyer who gave Rubio his first job as an attorney. “Marco can do left brain, so you get the logic, and he can do right brain, so you feel it in your heart and soul.”

This is Rubio’s first leadership audition, his chance to show Republicans that he’s more than a compelling speaker with boyish good looks, that he’s not just geographically, demographically and ideologically correct. Immigration, he says, is tricky. Though he has evolved on the issue—flip-flopped, if you prefer—he says he’s still not sure he’s got it exactly right. The word he uses most is balance. That was his first thought after he played that voice mail from his mother on his iPhone.

“I have to balance that humanity with reality,” he says. “We have immigration laws. They have to be followed. But yeah, she reminded me that there’s a human element to this as well. As a policymaker, you have to strike a balance.” The result will shape the future of the nation and the dreams of millions of immigrants, not to mention the fate of the GOP and the ambitions of Marco Rubio.

“It’s Complicated”
In his autobiography, An American Son, Rubio says his closest boyhood friend was his cigar-smoking, Fidel Castro–hating grandfather, Pedro Víctor García, a proud Cuban exile who taught him to believe in Reagan, American exceptionalism and himself. Hobbled by childhood polio, more book-smart than business-savvy, Papa never achieved much material success. But he spent hours with his grandson reading Spanish-language newspapers, fulminating about communism and pushing the future Senator to expand his horizons beyond football. “He would scold me for performing poorly in school, but he never let me believe I was incapable of being successful,” Rubio wrote. “His dreams for us were his legacy.”